In an automated information storage and retrieval system, also known as a library, numerous storage cells arrayed within the library are used to hold removable information media, such as magnetic tape cartridges or optical disk cartridges. (The term "cartridge" used herein refers to any housing structure for removable media; the present invention is not limited to a library for storing any particular type of media.) An accessor, furnished with a holding or gripping device and under the direction of a library controller, transports selected cartridges between storage cells and drive units. The library controller is interconnected with a host device, such as a mainframe or network computer, which issues control signals related to reading/writing data from/to selected cartridges. Library units also typically include an input/output station or port through which a system operator can pass a cartridge to be added to the storage array and through which the accessor can pass a cartridge to the operator for removal.
In larger library systems, an accessor with two or more grippers can be employed to increase the speed with which a desired cartridge is retrieved from a storage cell and mounted in a drive. In a library with a single-gripper accessor, when a drive unit has completed accessing data on a first cartridge and access to data on a second cartridge is desired, the process is as follows:
the accessor moves to the drive; PA1 the first cartridge is removed from the drive; PA1 the first cartridge is transported from the drive to an empty cell; PA1 the first cartridge is put into the empty cell; PA1 the accessor moves to the storage cell of the second cartridge; PA1 the second cartridge is retrieved from its cell; PA1 the second cartridge is transported to the drive; and PA1 the second cartridge is inserted in the drive. PA1 the accessor moves to the cell of the second cartridge; PA1 the second cartridge is retrieved by the first gripper; PA1 the second cartridge is transported from its cell to the drive; and PA1 the first and second cartridges are exchanged in the drive.
In a library with a multi-gripper accessor, the same process is as follows:
Upon completion of either of these two processes, the library controller obtains the next access command from a queue or directly from the host.
It will be appreciated that managing cartridges in a library with a multi-gripper accessor can be significantly faster than doing so in a library with just a single-gripper accessor.
In some libraries, each cartridge has an assigned, fixed "home" cell to which the cartridge is returned after use. In other libraries, cartridges do not have assigned, fixed home cells but, rather, are returned to any available, unoccupied cell, called a "floating home" cell. In combination with a multi-gripper accessor, cartridge exchanges can occur at cell locations as well as at drive locations, thereby reducing still further the time required to retrieve and mount cartridges.
While some small libraries have only one or two drive units and sixteen to sixty-four storage cells, larger libraries may have sixteen or more drives and 18,000 or more storage cells. There is a need, therefore, for improving the efficiency of accessor operations to decrease the amount of time between a data request being received from the host and the desired cartridge being mounted in a drive.